The permanent magnet motor can be classified into the sine-wave motor and the square-wave motor, according to its drive current and back-EMF waveform. The sine-wave permanent magnet motor is generally known as permanent magnet synchronous motor (PMSM), or the sine-wave AC servo motor. The square-wave permanent magnet motor is known as square-wave brushless DC motor (BLDCM).
During the 1980's, the square-wave permanent magnet motor was widely applied, whose external characteristics are almost the same as that of brush DC motor, and which is easy to control. But, it has a biggest disadvantage that there is great principle commutated torque ripple. Therefore, researchers put forward many compensation measures, whose effects in practical application were not satisfying.
As the torque ripple of the sine-wave permanent magnet motor is far smaller than that of the square-wave permanent magnet motor, during the 1990's, on the occasions of precision servo-drive application, the square-wave permanent magnet motor was gradually replaced by the sine-wave permanent magnet motor, which now has become the mainstream of today's industrial applications. However, the sine-wave permanent magnet motor will induce a great rise in complexity of the control system and a substantial increase of cost and what is more important is the great falling of the energy index of the motor.
On the other hand, the traditional square-wave brushless DC motor and its control technology are recognized as being mature. Due to the said disadvantage, its application is limited in occasions with lower demand, and the study on it has been rare at home and abroad.